1. Field of the Invention
This invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device having a reflectance control film formed on the end facet of an oscillator to enhance an output of an oscillated laser light.
2. Description of the Related Art
In a semiconductor laser device, it is well known in the art to form a reflectance control film on the end face of an oscillator in order to increase an output of an oscillated laser light. For example, the above reflectance control film may be formed by alternately laminating at least two thin films having different refractive indices so as to attain a large reflectance and increase the output of laser light. This type of semiconductor laser device is described in Japanese Patent Disclosure Nos. 64-33987 and 1-184893, for example.
A first thin film included in the thin films constituting the reflectance control film and formed in contact with the end facet of the oscillator is formed of material having a refractive index n of small value, and SiO.sub.2 (n is approx. 1.5), Al.sub.2 O.sub.3 (n is approx. 1.7) or Si.sub.3 N.sub.4 (n is approx. 1.8) is often used as the material, for example. A second thin film formed on the first thin film is formed of material having a large refractive index and amorphous silicon (n.gtoreq.3.2) is generally used. When the first thin film is formed of SiO.sub.2, the second thin film is formed of Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4. In order to form the first thin film, a vacuum deposition method (for SiO.sub.2) and a sputtering method (for Al.sub.2 O.sub.3, Si.sub.3 N.sub.4 or amorphous silicon) may be used.
In a general system using a semiconductor laser device, a photo detecting element for monitoring is disposed on the end facet of the oscillator opposite to the laser light emitting surface thereof and the laser is driven in an APC (Auto Power Control) manner according to a monitoring current derived from the photo detecting element in order to obtain a constant laser light output. That is, the monitoring photo detecting element is disposed on the side of the reflectance control film of the oscillator to receive a laser light which has passed the reflectance control film so as to generate a monitoring current. However, as the reflex index of the reflectance control film increases, the amount of laser light incident on the monitoring photo detecting element decreases. Therefore, in order to efficiently receive the laser light, the monitoring photo detecting element must be set closer to the reflectance control film. For example, in a case where a normal InGaAs-series PIN photodiode is disposed as the photo detecting element on the semiconductor laser device having a reflectance control film with a reflectance of approximately 80%, a distance between the reflectance control film and the light receiving surface of the photo detecting element must be set to be less than 1 mm in order to derive a monitoring current of more than 0.05 mA. This imposes a large limitation on the optical and mechanical design.
In a case where an amorphous silicon film is used as one of the thin films constituting the reflectance control film, the reflectance thereof may become larger than 80% even if it is a two-layered film, and therefore the limitation on the optical design becomes further severe. If the amorphous silicon film is formed by the vacuum deposition method, the control of the vapor deposition speed and the reflectance becomes difficult, making the film chemically and physically unstable. As a result, the reliability of the semiconductor laser device manufactured is lowered. Therefore, it is not practical to form the amorphous silicon film by use of the vacuum deposition method and it is a common practice to form the same by use of the sputtering method which is low in production efficiency than the vacuum deposition method.
It is also difficult to form an Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 film by use of the vacuum deposition method, and the refractive index thereof is as small as 1.7 to 1.8 and therefore the effect that the reflectance is improved cannot be obtained unless the thin films are laminated to make a four- or more-layered film. The total number of thin films formed is desired to be as small as possible when the uniformity of the film, stress between the films, manufacturing yield, forming time and the like are taken into consideration.